Moisture recycling is the atmospheric branch of the water cycle, including evaporation and precipitation. While the physical water cycle is well-understood, the social links among the recipients of precipitation back to the sources of evaporation are not. In this work, we develop a method to determine how these social connections unfold, using a mix of quantitative and qualitative methods, finding that there are distinct types of social connections with corresponding policy and management tools.

We present an "Earth observation-based" method for estimating root zone storage capacity – a critical parameter in land surface modelling that represents the maximum amount of soil moisture available for vegetation. Variability within a land cover type is captured, and a global model evaporation simulation is overall improved, particularly in sub-humid to humid regions with seasonality. This new method can eliminate the need for unreliable soil and root depth data in land surface modelling.

We investigate the temporal characteristics of partitioned evaporation on land, and we present STEAM (Simple Terrestrial Evaporation to Atmosphere Model) -- a hydrological land-surface model developed to provide inputs to moisture tracking. The terrestrial residence timescale of transpiration (days to months) has larger inter-seasonal variation and is substantially longer than that of interception (hours). This can cause differences in moisture recycling, which is investigated more in Part 2.

Satellite observations have increasingly been used for model calibration, while model structural developments largely rely on discharge data often resulting in poor representations of internal rainfall-runoff processes, especially for large river basins. This study explores the combined use of spatial pattern and temporal dynamics of satellite-based evaporation and total water storage data for model structural improvement and model calibration for a large, semi-arid and data scarce river system.

Precipitation over East Asia simulated in Met Office Unified Model is compared with observations. Moisture sources of EA precipitation are traced using a moisture tracking model. Biases in moisture sources are linked to biases in precipitation. By using the tracking model, changes in moisture sources can be attributed to changes in SST, circulation and associated evaporation. This proves that the method used in this study is useful to identify biases that cause biases in regional precipitation.

Tropical forest ecosystems are able to export a lot of water to the atmosphere by means of evaporation. However, little is known on how their complex structure affects this water flux. This paper analyzes the contribution of three canopy layers in terms of water fluxes and stable water isotope signatures. During the dry season in 2018 the two lower canopy layers provide 20 % of measured evaporation, highlighting the importance of knowing how forest structure can affect the hydrological cycle.

We quantify the differences in internal states and fluxes of twelve process-based models with similar streamflow performance and assess their plausibility using remotely-sensed estimates of evaporation, snow cover, soil moisture and total storage anomalies. The dissimilarities in internal process representation imply that these models cannot all simultaneously be close to reality. Therefore, we invite modelers to evaluate their models using multiple variables and to rely on multi-model studies.

In this paper we present a method to distribute crucial model parameters over subcatchments so as to enhance overall rainfall-runoff performance. We also analyse how soil moisture indicators can be used to distribute root zone moisture capacity over subcatchments. NDII proves to be very effective for this purpose, resulting in better overall model performance, good temporal correspondence between modelled soil moisture and the SWI, and improved recession behavior and dry season flow.

This work provides a global dataset on the fate of land evaporation for a fine-meshed grid of source and receptor cells. The dataset was created through a global run of the numerical moisture tracking model WAM-2layers.The dataset could be used for investigations into average annual, seasonal and inter-annual sink and source regions of atmospheric moisture from land masses for most of the regions in the world and comes along with example scripts for the readout and plotting of the data.

During rainfall events, the evaporation from Tropical forests is usually ignored. However, the water retained on the canopy during rainfall increases the evaporation despite the high humid conditions. In a Tropical wet forest in Costa Rica, it was possible to depict vapor plumes rising from the forest canopy during rainfall. These plumes are evidence of forest evaporation. Also, we identified those conditions that allowed this phenomenon to happen using time-lapse videos and meteorological data.

Natural systems evolve towards a state of maximum power, including estuarine circulation. The energy of lighter fresh water drives circulation, while it dissipates by friction. This rotational flow causes the spread of salinity, which is represented by the dispersion coefficient. In this paper, the maximum power concept provides a new equation for this coefficient. Together with the steady-state equation, this results in a new analytical model for density-driven salinity intrusion.

In the absence of discharge data in ungauged basins, remotely sensed river water level data, i.e. altimetry, may provide valuable information to calibrate hydrological models. This study illustrated that for large rivers in data scarce regions, river altimetry data from multiple locations combined with GRACE data have the potential to fill this gap when combined with estimates of the river geometry, thereby allowing a step towards more reliable hydrological modelling in data scarce regions.

Knowing the isotopic composition of water vapor in the air is a difficult task. The estimation of δ18O and δ2H has to be done carefully, because it is accompanied by a high risk of methodological errors (if it is sampled) or wrong assumptions that can lead to incorrect values (if it is modeled). The aim of this work was to compare available sampling methods for water vapor in the air and estimate their isotopic composition, comparing the results against direct measurements of the sampled air.

Tide–river dynamics play an essential role in large-scale river deltas as they exert a tremendous impact on delta morphodynamics, salt intrusion and deltaic ecosystems. For the first time, we illustrate that there is a critical river discharge, beyond which tidal damping is reduced with increasing river discharge, and we explore the underlying mechanism using an analytical model. The results are useful for guiding sustainable water management and sediment transport in tidal rivers.

Inside a wind tunnel we tested a novel measurement technique to measure wind speed. Our device can measure wind speed not only at one place, but it measures the wind speed along a fiber optic cable. This cable can be more then 1km in length and give thousands of measurements at once, which is interesting for locations/situations were the wind is not constant in space. Finally, we propose a method to estimate the precision which helps in designing future (outdoor) experiments.

Even models relying on physical laws have parameters that need to be measured or estimated. Thermodynamic optimality principles potentially offer a way to reduce the number of estimated parameters by stating that a system evolves to an optimum state. These principles have been applied successfully within the Earth system, but it is often unclear what to optimize and how. In this review paper we identify commonalities between different successful applications as well as some doubtful applications.

Supported by large-sample ecological observations, a novel, simple and topography-driven runoff generation module (HSC-MCT) was created. The HSC-MCT is calibration-free, and therefore it can be used to predict in ungauged basins, and has great potential to be generalized at the global scale. Also, it allows us to reproduce the variation of saturation areas, which has great potential to be used for broader hydrological, ecological, climatological, and biogeochemical studies.

We quantify net intercatchment groundwater flows in the Meuse basin in a complementary three-step approach through (1) water budget accounting, (2) testing a set of conceptual hydrological models and (3) evaluating against remote sensing actual evaporation data. We show that net intercatchment groundwater flows can make up as much as 25 % of mean annual precipitation in the headwaters and should therefore be accounted for in conceptual models to prevent overestimating actual evaporation rates.

Phosphorus (P) is important to global food security. Thus it is concerning that natural P reserves are predicted to deplete within the century. Here we explore the potential of P recovery from wastewater (WW) at global scale. We identify high production and demand sites to determine optimal market prices and trade flows. We show that 20 % of the agricultural demand can be met, yet only 4 % can be met economically. Nonetheless, this recovery stimulates circular economic development in WW treatment.

The measurement of stable isotopes in water vapor has been improved with the use of laser technologies. Its direct application in the field depends on the availability of infrastructure or the budget of the project. For those cases when it is not possible, we provide an alternative method to sample the air for its later measurement. This method is based on the use of a low-cost polyethylene bag, getting stable measurements with a volume of 450 mL of air reducing the risk of sample deterioration.

In many river basins, the development of hydrological models is challenged by poor discharge data availability and quality. In contrast, water level data are more reliable, as these are direct measurements and are unprocessed. In this study, an alternative calibration method is presented using water-level time series and the Strickler–Manning formula instead of discharge. This is applied to a semi-distributed rainfall-runoff model for the semi-arid, poorly gauged Mara River basin in Kenya.

Media play a key role in the communication between scientists and the general public. However, the interaction between scientists and journalists is not always straightforward. In this opinion paper, we present insights from hydrologists and journalists into the benefits, aftermath and potential pitfalls of science–media interaction. We aim to encourage scientists to participate in the diverse and evolving media landscape, and we call on the scientific community to support scientists who do so.

Moisture recycling is the atmospheric branch of the water cycle, including evaporation and precipitation. While the physical water cycle is well-understood, the social links among the recipients of precipitation back to the sources of evaporation are not. In this work, we develop a method to determine how these social connections unfold, using a mix of quantitative and qualitative methods, finding that there are distinct types of social connections with corresponding policy and management tools.

Around half of the carbon that humans emit into the atmosphere each year is taken up on land (by trees) and in the ocean (by absorption). We construct a simple model of carbon uptake that, unlike the complex models that are usually used, can be analysed mathematically. Our results include that changes in atmospheric carbon may affect future carbon uptake more than changes in climate. Our simple model could also study mechanisms that are currently too uncertain for complex models.

This paper provides the connection between two simple equations describing groundwater flow at different scales: the Darcy equation describes groundwater flow at pore scale, the linear reservoir equation at catchment scale. The connection between the two appears to be very simple. The two parameters of the equations are proportional, depending on the porosity of the subsoil and the resistance for the groundwater to enter the surface drainage network.

This paper presents a new equation for the dispersion of salinity in alluvial estuaries based on the maximum power concept. The new equation is physically based and replaces previous empirical equations. It is very useful for application in practice because in contrast to previous methods it no longer requires a calibration parameter, turning the method into a predictive method. The paper presents successful applications in more than 23 estuaries in different parts of the world.

Phosphorus is necessary for the development of crops and is therefore essential in safeguarding our food security. Several studies predict that our rock phosphate reserves, used to create synthetic, phosphatic fertilizers, may become depleted within this century. This study roughly approximates for which areas in Africa we can instead recover phosphorus from wastewater in order to reduce our dependancy on unsustainable rock phosphate.

Using the DTS technology, we measured the evaporation of a forest using fibre optic cables. The cables work like long thermometers, with a measurement every 12.5 cm. We placed the cables vertically along the tower, one cable being dry, the other kept wet.
By looking at the dry and wet cable temperatures over the height we are able to study heat storage and the amount of water the forest is evaporating. These results can be used to better understand the storage and heat exchange of forests.

At larger scales, the flow of rivers can often be described by a relatively simple, exponential decay, and it is unclear how such simple behaviour can be explained given that river basins show such vast complexity. Here, we use a highly idealised model to show that such simple behaviour can be explained by viewing it as the emergent consequence of the groundwater system (which feeds river flow) minimising its energy dissipation.

Large-scale hydrological variability can affect society in profound ways; floods and droughts, for example, often cause major damage and hardship. A recent gathering of hydrologists at a symposium to honor the career of Professor Eric Wood motivates the present survey of recent research on this variability. The surveyed literature and the illustrative examples provided in the paper show that research into hydrological variability continues to be strong, vibrant, and multifaceted.

An estuary is where fresh water rivers meet the salty open sea. This mixture of salty fresh water leads to a varying water quality. There a model works well showing how far the salty water can travel, with an empirical parameter that needs to be calibrated every time. This paper provides a possible solution for this parameter to make the model predictive. Also, the model was improved by considering 2-D exchange flow. This new model was supported by observations in 18 estuaries around the world.

The natural environment that we live in is the result of evolution. This does not only apply to ecosystems, but also to the physical environment through which the water flows. This has resulted in the formation of flow patterns that obey sometimes surprisingly simple mathematical laws. Hydrological models should represent the physics of these patterns and should account for the fact that the ecosystem adjusts itself continuously to changing circumstances. Physics-based models are alive!

This research seeks out to answer a fundamental question about the functioning of the water cycle in the atmosphere: how much time does a water particle spend in the atmosphere? Based on state-of-the-art data, we derive a global average residence time of water in the atmosphere of 8–10 days. We further show in this paper how the residence time of water varies in time and space. This serves to illustrate why it is so difficult to make weather predictions on timescales longer than a week.

In this study, the rainfall–runoff models of eight international research groups were compared for a set of subcatchments of the Meuse basin to investigate the influence of certain model components on the modelled discharge. Although the models showed similar performances based on general metrics, clear differences could be observed for specific events. The differences during drier conditions could indeed be linked to differences in model structures.

The core component of many hydrological systems, the moisture storage capacity available to vegetation, is typically treated as a calibration parameter in hydrological models and often considered to remain constant in time. In this paper we test the potential of a recently introduced method to robustly estimate catchment-scale root-zone storage capacities exclusively based on climate data to reproduce the temporal evolution of root-zone storage under change (deforestation).

A comprehensive and detailed data set of the salinity distribution over an entire year in a complex and dynamic (because heavily utilized and modified) deltaic river system was thoroughly analysed, and formed the basis for a validated analytical model that can predict the extent of seawater among other salinity sources in an estuary. The procedure can be applied to other estuaries.

We demonstrated that the readily available NDII remote sensing product is a very useful proxy for moisture storage in the root zone of vegetation. We compared the temporal variation of the NDII with the root zone storage in a hydrological model of eight catchments in the Upper Ping River in Thailand, yielding very good results. Having a reliable NDII product that can help us to estimate the actual moisture storage in catchments is a major contribution to prediction in ungauged basins.

Maharashtra is one of the states in India that has witnessed highest rates of farmer suicides as proportion of total number of suicides. We interpret the crisis using a socio-hydrological model in two adjoining regions in Maharashtra, Marathwada and Desh, with higher farmer suicide rates in the former. The analysis confirms existing narratives: low (soil) water storage capacities, no irrigation and access to alternative sources of incomes are to blame for the crisis.

We present an "Earth observation-based" method for estimating root zone storage capacity – a critical parameter in land surface modelling that represents the maximum amount of soil moisture available for vegetation. Variability within a land cover type is captured, and a global model evaporation simulation is overall improved, particularly in sub-humid to humid regions with seasonality. This new method can eliminate the need for unreliable soil and root depth data in land surface modelling.

In this paper, an analytical model for tide-river dynamics has been used to understand the influence of tide and fresh water discharge on the rise of mean water level along the estuary, which remains poorly understood. It is shown that the mean water level is influenced primarily by the tide-river interaction in the tide-dominated region, while it is mainly controlled by the river flow in the upstream part of the estuary.

The heterogeneity of landscapes in river basins strongly affects the hydrological response. In this study, the distributed mesoscale Hydrologic Model (mHM) was equipped with additional processes identified by landscapes within one modelling cell. Seven study catchments across Europe were selected to test the value of this additional sub-grid heterogeneity. In addition, the models were constrained based on expert knowledge. Generally, the modifications improved the representation of low flows.

We revised the predictive equations for two calibrated parameters in salt intrusion model (the Van der Burgh coefficient K and dispersion coefficient D) using an extended database of 89 salinity profiles including 8 newly conducted salinity measurements. The revised predictive equations consist of easily measured parameters such as the geometry of estuary, tide, friction and the Richardson number. These equations are useful in obtaining the first estimate of salinity distribution in an estuary.

Phosphate rock is a finite resource required for fertilizer production. Following a debate over the PR depletion timeline, global PR reserves were recently increased 4-fold based mainly on a restatement of Moroccan reserves. We review whether this restatement is methodologically compatible with resource terminology used in major resource classifications, whether resource classification nomenclature is sufficiently understood in the literature, and whether the recent restatements are reliable.

We investigate the temporal characteristics of partitioned evaporation on land, and we present STEAM (Simple Terrestrial Evaporation to Atmosphere Model) -- a hydrological land-surface model developed to provide inputs to moisture tracking. The terrestrial residence timescale of transpiration (days to months) has larger inter-seasonal variation and is substantially longer than that of interception (hours). This can cause differences in moisture recycling, which is investigated more in Part 2.

Global-scale hydrologic forecasts should account for attenuation through lakes and reservoirs. There is no consensus on the best approach to estimating this attenuation in large-spatial-scale hydrologic forecasts. This article investigates two existing parsimonious approaches to estimating reservoir outflows. We test each method at 60 reservoirs in the United States. We find that a method first developed in 2003 can provide a reasonable approximation of diurnal reservoir outflows.

We evaluate the soil moisture response in the humid tropics to El Niño during the three most recent super El Niño events. Our estimates are compared to in situ soil moisture estimates that span five continents. We find the strongest and most consistent soil moisture decreases in the Amazon and maritime southeastern Asia, while the most consistent increases occur over eastern Africa. Our results can be used to improve estimates of soil moisture in tropical ecohydrology models at multiple scales.

The Budyko framework which relies on the supply and demand concept could be effectively adapted and extended to quantify the role of drivers – both changing climate and local human disturbances – in altering the land-surface response. This framework is extended with a few illustrative examples for quantifying the variability in land-surface fluxes for natural and human-altered watersheds. Potential for using observed and remotely sensed datasets in capturing this variability is also discussed.

Evapotranspiration (ET) rates and properties that regulate them are spatially heterogeneous. Averaging over spatial heterogeneity in precipitation (P) and potential evapotranspiration (PET) as the main drivers of ET may lead to biased estimates of energy and water fluxes from the land to the atmosphere. We show that this bias is largest in mountainous terrains, in regions with temperate climates and dry summers, and in landscapes where spatial variations in P and PET are inversely correlated.

Evapotranspiration (ET) links global water, carbon and energy cycles. We used 4 remote sensing models, 2 machine-learning algorithms and 14 land surface models to analyze the changes in global terrestrial ET. These three categories of approaches agreed well in terms of ET intensity. For 1982–2011, all models showed that Earth greening enhanced terrestrial ET. The small interannual variability of global terrestrial ET suggests it has a potential planetary boundary of around 600 mm yr-1.

The combined use of GRACE mass anomalies and observed river discharge for the first time allows us to quantify the water storage volumes drainable by gravity on global scales. Modelling of catchment and river network storages in a cascade with different dynamics reveals the time lag between total mass and runoff is caused by a non-zero river network storage. This allows catchment and river network storage volumes to be distinguished and is thus of great importance for water resources management.

How far can we reach in predicting river flow globally, using integrated catchment modelling and open global data? For the first time, a catchment model was applied world-wide, covering the entire globe with a relatively high resolution. The results show that stepwise calibration provided better performance than traditional modelling of the globe. The study highlights that open data and models are crucial to advance hydrological sciences by sharing knowledge and enabling transparent evaluation.

Focusing on the multifaceted nature of droughts, this study quantifies the change in global drought risks for 1.5 and 2.0 °C warming trajectories by a multi-model ensemble under three representative concentration pathways (RCP2.6, 4.5 and 8.5). Socioeconomic exposures are investigated by incorporating the dynamic shared socioeconomic pathways (SSPs) into the drought impact assessment. The results show that even the ambitious 1.5 °C warming level can cause substantial increases on the global scale.

Poorly monitored river flows in many regions of the world have been hindering our ability to accurately estimate global water usage. In this paper we present a method to derive continuous records of streamflow from a set of in situ gauges. Applying this method to the Ohio River basin, we found that we could reliably generate estimates of streamflow throughout the basin using only a small set of streamflow gauges, which can be useful for global river basins where we do not have good observations.

A new approach for regional rainfall–runoff modeling using long short-term memory (LSTM)-based models is presented and benchmarked against a range of well-known hydrological models. The approach significantly outperforms regionally calibrated hydrological models but also basin-wise calibrated models. Furthermore, we propose an adaption of the LSTM that allows us to extract the learned catchment understanding of the model and show that it matches our hydrology expert knowledge.

Over the past decades, changes in land use and climate over Europe have impacted the average flow of water flowing through rivers and reservoirs (the so-called water yield). We quantify these changes using a simple but widely tested modelling approach constrained by observations of lysimeters across Europe. Results show that the contribution of land use to changes in water yield are of the same order as changes in climate, showing that impacts of land use changes cannot be neglected.

Comprehensive characterization of extreme drought events in the Amazon is provided with respect to their cause, type, spatial extent, and impact on different water stores. Basin-averaged trends in water storage indicate that the Amazon is getting wetter; however its southern and southeastern portions are getting drier. Water deficit is found to be 3-fold higher than the total water supplied during some drought years. Water deficit due to low precipitation events is absorbed by the groundwater.

Human activities associated with water resource management have significantly increased in China during the past decades. This assessment helps us understand how streamflow has been affected by climate and human activities in China. Our analyses indicate that the climate impact has dominated streamflow changes in most areas, and human activities (in terms of water withdrawals) have increasingly decreased streamflow in the northern basins of China which are vulnerable to future climate change.

This study investigates the value of assimilating coarse-resolution remotely sensed soil moisture data into high-resolution land surface models for improving soil moisture and runoff modeling. The soil moisture estimates in this study, with complete spatio-temporal coverage and improved spatial resolution from the assimilation, offer a new reanalysis product for the monitoring of surface soil water content and other hydrological fluxes at 3 km resolution over Europe.

Providing reliable estimates of water fluxes at the continental scale is challenging. We extended a regional hydrological model to the entirety of South America and assessed its performance using multiple observations. After a comparison with global models, we show the extent to which estimates of daily river discharge can be improved, even by using global forcing data. Issues of global-/continental-scale modeling and future directions for simulating discharge in this continent are discussed.

In high-latitude (> 60° N) watersheds, measuring the snowpack and predicting of snowmelt runoff are uncertain due to the lack of data and complex physical processes. This provides challenges for hydrological assessment and operational water management. Global re-analysis datasets have great potential to aid in snowpack representation and snowmelt prediction when combined with a distributed hydrological model, though they still have clear limitations in remote boreal forest and tundra environments.

Drought is a natural hazard that has severe environmental and socioeconomic impacts around the globe. Here, we quantified the time taken for drought to propagate from precipitation droughts to soil moisture and streamflow droughts. Results show that propagation timescales are strongly related to climate type, with fast responses in tropical regions and slow responses in arid regions. Insight into the timescales of drought propagation globally may help improve seasonal drought forecasting.

We assimilated three satellite soil moisture retrievals based on different microwave frequencies into a hydrological model. Two sets of experiments were performed, first assimilating the retrievals individually and then assimilating each set of two retrievals jointly. Overall, assimilation improved agreement between model and field-measured soil moisture. Joint assimilation resulted in model performance similar to or better than assimilating either retrieval individually.

In this study, we adjust a simple hydrological model to several observational datasets, including satellite observations of the land's total water storage. We apply the model to northern latitudes and find that the dominating factor of changes in the total water storage depends on both the spatial and temporal scale of analysis. While snow dominates seasonal variations, liquid water determines year-to-year variations, yet with increasing contribution of snow when averaging over larger regions.

This work improves river discharge estimation by taking advantages of observation and model simulations. The new estimation takes into account both gauged and un-gauged rivers, and it compensates model systematic errors and missing processes (e.g., human water usage). This improved estimation is important not only for water resources management and ecosystem health over continent but also for ocean dynamics and salinity.

The petabytes of existing global environmental data provide an invaluable asset to improve the characterization of land heterogeneity in Earth system models. This study introduces a clustering algorithm that summarizes a domain's heterogeneity through spatially interconnected clusters. A series of land model simulations in central California using this approach illustrate the critical role that multi-scale heterogeneity can have on the macroscale water, energy, and carbon cycles.

To ensure a sustainable supply of groundwater, scientific information about what is going into the system as recharge and what is taken out of the system via pumping is essential. This study identified the most influential factors in groundwater recharge and developed an empirical global recharge model. The meteorological and vegetation factors were the most important factors, and the long-term global average recharge was 134 mm per year. This model will aid in groundwater policy-making.

A new product (Global Forcing Data, GFD) that provides bias-adjusted meteorological forcing data for impact models, such as hydrological models, is presented. The main novelty with the product is the near-real time updating of the data which allows more up-to-date impact modeling. This is performed by combining climatological data sets with climate monitoring data sets. The potential in using the data to initialize hydrological forecasts is further investigated.

Global hydrological models aim to model hydrological processes, like flows in a river, on a global scale, as opposed to traditional models which are regional. A big challenge in creating these models is the inclusion of impacts on the hydrological cycle caused by humans, for example by the operation of large (hydropower) dams. The presented study investigates a new way to include these impacts by dams into global hydrological models.

Six schemes were added to the H08 global hydrological model (GHM) to represent human water abstraction more accurately and ensure that all water fluxes and storage are traceable in each grid cell at a daily interval. The schemes of local reservoirs, aqueduct water transfer, and seawater desalination were incorporated into GHMs for the first time, to the best of our knowledge. H08 has become one of the most detailed GHMs for attributing water sources available to humanity.

A global data record for all four terrestrial water budget variables (precipitation, evapotranspiration, runoff, and total water storage change) at 0.5° resolution and monthly scale for the period of 1984–2010 is developed by optimally merging a series of remote sensing products, in situ measurements, land surface model outputs, and atmospheric reanalysis estimates and enforcing the mass balance of water. Initial validations show the data record is reliable for climate related analysis.

Drought can affect large regions of the world, implying the need for a global monitoring tool. For the JRC Global Drought Observatory (GDO,
http://edo.jrc.ec.europa.eu/gdo/), 3 soil moisture anomaly datasets have been compared, in order to evaluate their consistency. The analysis performed on five macro-regions (North America, Europe, India, southern Africa and Australia) suggests the need to combine these different data sources in order to obtain robust assessments over a variety of conditions.

We investigate how changes in land cover, such as deforestation, affect river runoff and evaporation from the land surface. We use computer simulations to show that the impact of land cover changes is significant and, when globally averaged, it is as important as more direct human impacts through water use (such as irrigation). There is large spatial variability in the impact of land cover change, with the largest changes when tall vegetation (such as forests) is replaced by crop fields.

Providing large-scale (regional or global) simulation of floods at fine spatial resolution is difficult due to computational constraints but is necessary to provide consistent estimates of hazards, especially in data-scarce regions. We assessed the capability of an advanced global-scale river model to simulate an extreme flood at fine resolution. We found that when multiple flow connections in rivers are represented, the model can provide reliable fine-resolution predictions of flood inundation.

Bias correction of climate model outputs has become a standard procedure accompanying climate change impact studies. However, it introduces a new level of uncertainty in the modelling chain which remains relatively unexplored. In this work we present a new framework for the quantification and categorization of the effect of bias correction on global hydrological simulations and we derive information on the sensitivity and magnitude of the effect of GCM biases on runoff, at the global scale.

We inspect the state-of-the-art of several land surface (LSMs) and hydrologic models (HMs) and show that most do not have consistent and realistic parameter fields for land surface geophysical properties. We propose to use the multiscale parameter regionalization (MPR) technique to solve, at least partly, the scaling problem in LSMs/HMs. A general model protocol is presented to describe how MPR can be applied to a specific model.

Rapidly increasing population and human activities have altered terrestrial water fluxes on an unprecedented scale. Awareness of potential water scarcity led to first global water resource assessments; however, few hydrological models considered the interaction between terrestrial water fluxes and human activities. Our contribution highlights the importance of human activities transforming the Earth's water cycle, and how hydrological models can include such influences in an integrated manner.

This paper presents a skill assessment of the global seasonal streamflow forecasting system FEWS-World. For 20 large basins of the world, forecasts using the ESP procedure are compared to forecasts using actual S3 seasonal meteorological forecast ensembles by ECMWF. The results are discussed in the context of prevailing hydroclimatic conditions per basin. The study concludes that in general, the skill of ECMWF S3 forecasts is close to that of the ESP forecasts.

Runoff measurements for 966 catchments around the globe were used to assess the quality of the daily runoff estimates of 10 hydrological models run as part of tier-1 of the eartH2Observe project. We found pronounced inter-model performance differences, underscoring the importance of hydrological model uncertainty.

Our study aims to explore and understand the physical controls on spatial patterns of pan-European flow signatures by taking advantage of large open datasets. Using tools like correlation analysis, stepwise regressions and different types of catchment classifications, we explore the relationships between catchment descriptors and flow signatures across 35 215 catchments which cover a wide range of pan-European physiographic and anthropogenic characteristics.

Riparian wetlands are disappearing worldwide due to altered river flow regimes. The WaterGAP3 modeling framework is used to compare modified to natural flow regimes at 93 Ramsar sites. Results indicate that water resource management seriously impairs inundation patterns at 29 % of the sites. New dam initiatives are likely to affect especially wetlands located in South America, Asia, and the Balkan Peninsula. Hotspots for climate change impacts could be eastern Europe and South America.

Agricultural production in the Ganges River basin is affected by the water shortage in the dry months, while the excess water during the rainy season causes floods in the downstream. Annual total surface runoff generated in the basin is about 298 ± 99 Bm3, and runoff in the monsoon months contributes up to 80 % of this total runoff. Comparison of sub-basin-wise surface runoff with the estimated unmet water demand indicated that capturing only a portion of the wet-season runoff would be sufficient.

Upscaling instantaneous to daily evapotranspiration (ETi–ETd) is one of the central challenges in regional vegetation water-use mapping using polar orbiting satellites. Here we developed a robust ETi upscaling for global studies using the ratio between daily and instantaneous global radiation (RSd/RSi). Using data from 126 FLUXNET tower sites, this study demonstrated the RSd/RSi ratio to be the most robust factor explaining ETd/ETi variability across variable sky conditions and multiple biomes.

Modelling inundations is pivotal to assess current and future flood hazard, and to define sound measures and policies. Yet, many models focus on the hydrologic or hydrodynamic aspect of floods only. We combined both by spatially coupling a hydrologic with a hydrodynamic model. This way we are able to balance the weaknesses of each model with the strengths of the other. We found that model coupling can indeed strongly improve discharge simulation, and see big potential in our approach.

Virtual water trade (VWT) embedded in crop trade is an important component of water management. Vulnerable importers in VWT were classified through connectivity and volume of VWT using degree centrality of a VWT network. Influential traders on entire VWT were classified through eigenvector centrality of a VWT network.

Although seawater desalination has been widely implemented and used as a key source of water in arid regions, it has been seldom included in global water resource assessments based on numerical simulations. We first developed a global model to estimate the areal extent and production of seawater desalination which was designed to be incorporated with global hydrological models. The model was applied to future periods under three distinct socioeconomic conditions.

The western African Sahel region has been one of the most important research areas for studying climatic variability due to its fragile climate conditions. Between 1950 and 2012, summer rainfall in the Sahel changed from a multi-decadal decreasing trend to an increasing trend (positive trend shift) in the mid-1980s. We found that this trend shift was synchronous with similar trend shifts in global oceanic evaporation and in land precipitation on all continents except the Americas.

Impacts of climate change on hydropower potential of China are investigated using projections from multiple general circulation models and global hydrological models. Results show that the projected total hydropower potential of China generally increases (e.g., in southwest China) while the maximum production of current hydropower stations may decrease (e.g., in Sichuan and Hubei provinces) in the future. This study prompts the consideration of climate change in hydropower planning in China.

We statistically modeled surface water extent (SWE) and inundation dynamics from a unique Landsat-based time series (1986–2011) for Australia's Murray–Darling Basin as a function of river flow and spatially explicit time series of rainfall, evapotranspiration and soil moisture. We present a data-driven and transferable approach that allowed us to model SWE through periods of flooding and drying for 363 floodplain units and to identify local combinations of variables that drive SWE dynamics.

A set of the new Euro-CORDEX projections is used to examine the impact of high-end scenarios on water availability and stress at a pan-European scale. Drought climatology is investigated for five major European basins along with the impact of +2 °C versus +4 °C warming. The effect of bias correction is also examined. The selection of the observational data set used for bias adjustment has an impact on the projected signal that could be of the same order of magnitude as the selection of the RCM.

We present an "Earth observation-based" method for estimating root zone storage capacity – a critical parameter in land surface modelling that represents the maximum amount of soil moisture available for vegetation. Variability within a land cover type is captured, and a global model evaporation simulation is overall improved, particularly in sub-humid to humid regions with seasonality. This new method can eliminate the need for unreliable soil and root depth data in land surface modelling.

A meta-analysis on 192 peer-reviewed articles reporting applications of a land surface model in a distributed way reveals that the spatial resolution at which the model is applied has increased over the years, while the calibration and validation time interval has remained unchanged. We argue that the calibration and validation time interval should keep pace with the increase in spatial resolution in order to resolve the processes that are relevant at the applied spatial resolution.

This original paper provides a guideline to select statistical downscaling methods (SDMs) in climate change impact studies (CCIS) to minimize uncertainty from downscaling. Three SDMs were applied to NCEP reanalysis and 2 GCM data values. We then analyzed the sensitivity of the hydrological model to the various downscaled data via 5 hydrological indicators representing the main features of the hydrograph. Our results enable selection of the appropriate SDMs to be used to build climate scenarios.

The WACMOS-ET project aims to advance the development of land evaporation estimates on global and regional scales. Evaluation of current evaporation data sets on the global scale showed that they manifest large dissimilarities during conditions of water stress and drought and deficiencies in the way evaporation is partitioned into several components. Different models perform better under different conditions, highlighting the potential for considering biome- or climate-specific model ensembles.

In this study a common reference input data set from satellite and in situ data is used to run four established evapotranspiration (ET) algorithms using sub-daily and daily input on a tower scale as a testbed for a global ET product. The PT-JPL model and GLEAM provide the best performance for satellite and in situ forcing as well as for the different temporal resolutions. PM-MOD and SEBS perform less well: the PM-MOD model generally underestimates, while SEBS generally overestimates ET.

Giordano, M., Suhardiman, D., and Peterson-Perlman, J.: Do hydrologic rigor
and technological advance tell us more or less about transboundary water
management?, Int. Environ. Agreem.-P., 16, 815–831, https://doi.org/10.1007/s10784-015-9297-2, 2015. a

Hoekstra, A. Y.: The Global Dimension of Water Governance: Why the River Basin
Approach Is No Longer Sufficient and Why Cooperative Action at Global Level
Is Needed, Water, 3, 21–46, https://doi.org/10.3390/w3010021, 2010. a

Kustu, M. D., Fan, Y., and Robock, A.: Large-scale water cycle perturbation
due to irrigation pumping in the US High Plains: A synthesis of observed
streamflow changes, J. Hydrol., 390, 222–244,
https://doi.org/10.1016/j.jhydrol.2010.06.045, 2010. a

Kustu, M. D., Fan, Y., and Rodell, M.: Possible link between irrigation in the
U.S. High Plains and increased summer streamflow in the Midwest, Water
Resour. Res., 47, W03522, https://doi.org/10.1029/2010WR010046, 2011. a, b

Winds carry air moisture from one place to another. Thus, land-use change that alters air moisture content can also modify downwind rainfall and distant river flows. This aspect has rarely been taken into account in studies of river flow changes. We show here that remote land-use change effect on rainfall can exceed that of local, and that foreign nation influence on river flows is much more prevalent than previously thought. This has important implications for both land and water governance.

Winds carry air moisture from one place to another. Thus, land-use change that alters air...